U.S. Pat. No. 3,692,184, for example, shows a filter pack of this general type in which folds of filter paper are held in position by bands of adhesive material entering the intervening spaces A similar device is disclosed in German printed specification No. 1,093,188 of Nov. 17, 1960; there a pleated sheet of filter paper is provided on opposite sides with two sets of parallel, spaced-apart adhesive threads which hold the pleats together after stacking and compression. The adhesive threads also act as spacers maintaining a certain clearance between adjacent pleats so as to increase the dust-absorbing capacity of the filter pack.
In commonly owned German printed specification No. 28 35 181, published Jan. 31, 1980, an improved filter assembly of this type has been disclosed in which the adhesive threads of the earlier device are replaced by retaining strips of inherently nonbonding character between which an undulating filter sheet is sandwiched. These strips serve as carriers for a bonding agent which, however, does not penetrate between closely juxtaposed legs of integrally interconnected U-shaped pleats that are formed by the strips and are interspersed with the sheet undulations. Instead, the adhesive merely fills the gaps between the bights of adjoining U-shaped pleats of the strips which are thereby rigidified to form two relatively staggered sets of multiprong clamps bracketing and gripping the undulating filter sheet from opposite sides. The filter sheet is described as consisting of an elastically compressible fleece of filamentary material, specifically spun glass fibers, which resembles cotton wadding and may be stabilized by a glass-fiber netting on its surfaces. The strips are said to consist of a less compressible fleece, e.g. of polyethylene fibers.
In order to apply the adhesive to the retaining strips of such a filter pack, the undulating sheet with folded-in strip zones must first be laid on a table whereupon the bights of the strips exposed at the upper surface of the assembly can be coated with the adhesive. After the bonding agent has hardened along that surface, the assembly is reversed and the remaining bights are similarly coated. Attempts to cement the strips to the glass-fiber fleece have proven unsuccessful because the adhesive is rapidly absorbed into the interstices of the fleece. Thus, the prongs of the clamps formed by the rigidified strips are held in position between the sheet undulations essentially by the surface roughness of the contacting filamentary members.